1998 Volkswagen Polo kangarooing when accelarating uphill

My car was functioning absolutely fine one minute then as i was going uphill on the way home, it started kangarooing, its fine on generally flat surfaces although does have the occasional kangarooing. Has been having the same problem since. Called out breakdown guys, says it could be a problem with either electrics in the engine ie loose or faulty wire or engine, now the problem is the engine when stationary is still very quiet although every so often it jolts as if it wants to cut out. it has had new air, oil filters and new oil 4000 miles ago, but nothing else on an internal state to do with the front of the car. could it be plugs at all. I would be greatful if anyone could give me a suggestion